The fuel vapor treatment system restricts the dissipation of fuel vapor produced in a fuel tank to the atmosphere. A fuel vapor introduced into the system from the fuel tank through an inlet passage is once adsorbed into an adsorbing material disposed within a canister and, when an internal combustion engine operates, the adsorbed fuel vapor is purged to an intake pipe in the internal combustion engine through a purging passage by utilizing a negative pressure developed within the intake pipe. The adsorption capacity of the adsorbing material is recovered by purging of the fuel vapor. Purging of the fuel vapor is performed by metering the flow rate of purged gas (the flow rate of purged air and that of purged fuel vapor) which metering is performed by a purge control valve disposed in the purging passage.
The purged fuel vapor burns together with fuel which is fed from an injector, therefore, in order to attain an appropriate air/fuel ratio, it is important to measure an actual amount of purged fuel vapor with a high accuracy. As a method for measuring the purge quantity, a method wherein a hot wire type mass flow meter is installed in a purging passage is disclosed in JP-5-18326A.
However, the flow meter is generally designed and calibrated on the premise of 100% air gas or a gas of a single component. Therefore, it has been difficult to measure with a high accuracy the flow rate of an air-fuel vapor mixture of which concentration is not constant like the purged gas. In JP-5-33733A (U.S. Pat. No. 5,216,995), another hot wire type mass flow meter is installed in an atmosphere passage which branches from the purging passage and the volume flow rate of the purged gas and the concentration of fuel vapor in the purged gas are detected from output values provided from the two mass flow meters.
In JP-5-18326A and JP-5-33733A (U.S. Pat. No. 5,216,995), since the flow meter(s) is installed in the purging passage, the concentration of fuel vapor cannot be detected unless purging of fuel vapor is performed with flow of purged gas. Therefore, for reflecting a measured concentration of fuel vapor in the control of air-fuel ratio, it is necessary to measure the concentration of fuel vapor before the purged fuel vapor reaches the injector position, and to correct a command value for the amount of fuel to be injected from the injector based on the measured concentration of fuel vapor.
However, in the case of an engine having a small intake pipe volume or in an operation region of a high flow velocity of intake air, the time required for purged fuel vapor to reach the injection position is shorter than the time required for completing the measurement of a fuel vapor concentration and thus it is hard to reflect a properly measured fuel vapor concentration in the control of air-fuel ratio. Alternatively, the engine structure including the layout of pipes, and the purge starting operation region are restricted. At present, throttling the purge flow rate up to the extent that the fuel vapor does not exert a bad influence on the control of air-fuel ratio is the only way to avoid the influence of variation in the concentration of fuel vapor. Without purge restriction, it is difficult to control the air-fuel ratio properly. Particularly, when a fuel vapor treatment system is to be applied to a hybrid vehicle which has recently been spotlighted, it is absolutely necessary to carry out a large quantity purge for the recovery of adsorption capacity because of the opportunity of purging is limited. It is expected to develop a technique which can measure an actual purge quantity of fuel vapor with a high accuracy and increase the purge flow rate.